nuke reactor circulation pumps

[p.larner]

is there any reason the pumps need power from outide and not use the power the plant produces,apart from cost?.

[jbb]

Buckle up! This could get bumpy.

As I understand it, you need to consider cases where something has gone wrong and the turbine has ‘tripped,’ ie disconnected from the grid and stopped generating.

Alternatively, we could think about refueling stops, where there’s no generation happening but you still need to cool the reactor core. They apparently generate a lot of heat even after all the fission reactions have stopped, because the immediate products of the fission reactions are very radioactive (and hence generate lots of heat).

[CatalinaWOW]

It is my understanding that most of them do use internally generated power, with multiple back ups like grid sources and on site gas or diesel generators to deal with situations jbb mentioned and many more. 

[Andy Chee]

Using Fukushima as an example.  Immediately upon earthquake detection, all the control rods dropped into place and shutdown the nuclear reaction and power generation.

Despite shutdown, the core remains hot, much like a car engine that has just had its ignition key removed a minute ago.  Unlike a car engine though, the Fukushima reactor is unable to dissipate the heat naturally, and so requires cooling pumps.

But with the reactors shutdown, how do you power the cooling pumps?  You use either or both, grid power or onsite diesel generators.  In the case of Fukushima, the tsunami wave that arrived an hour after the earthquake, flooded and destroyed the switch gear for both.

Evidently an hour of cooling is not enough.  The shutdown reactors remained hot enough to split water into hydrogen & oxygen.  The hydrogen production could not be contained, the hydrogen ignited, and blew the tops off the buildings (though the reactor pressure vessels & concrete radiation containment remained intact).

In summary, external power for cooling systems is mandatory for a nuclear power station (either grid power or diesel generators).  You can't rely on a shutdown core to generate enough power for the cooling pump.

Incidentally, there are newer Generation IV (four) reactor designs which use passive cooling techniques.  These new reactor designs could potentially do away with external power.  The Fukushima reactors are Generation II (two) design.

[floobydust]

Power plants require grid power to startup and run, usually called "station power". Ones I worked in (coal, natural gas) had separate lower voltage feed like 25kV bus lines from another grid interconnect. They did not power themselves, the output was 240kV or 500kV. You need power for plant SCADA and control, to operate valve actuators etc.

Reference the Fukushima nuclear disaster where the earthquake scuttled grid power lines, total loss of plant power.
Leaving only the emergency diesel generators but of course they were soaked with seawater by the tsunami and could not start, as well as the switchgear flooded. I'm not sure of the role of the DC battery system.

[Andy Chee]

As a slight tangent, you might be interested to know that even regular non-nuclear power stations require a source of external power. 

For example this turbine explosion was also a result of loss of external power:

https://www.abc.net.au/news/2021-06-02/qld-cs-energy-releases-photo-of-damaged-callide-power-station/100186330

[moffy]

Another nuclear power station event, Chernobyl, there is a computer simulator you can download to reconstruct the accident:

The video describes in detail what happened, and it looks like that even a shut down reactor has to deal with many MWs of thermal energy.

[Someone]

it looks like that even a shut down reactor has to deal with many MWs of thermal energy.

Somewhere around 6% of the thermal dissipation compared to full operation:
https://en.wikipedia.org/wiki/Decay_heat

[Jeroen3]

The scale of the systems required for the generators used in power plants requires more than 24V backup.
Each panel has that, but there also is a backup bus and emergency bus.

For example, you need to operate valves, fuel pumps, louvres, fans. 24V isn't cutting it.
Also lighting and pc's for the crew to command and control what is going on.

Then there is the problem that exciting these large machines so that they begin generating voltage requires external energy.
They're not self-starting, even if you get them spinning!

[Ranayna]

Despite shutdown, the core remains hot, much like a car engine that has just had its ignition key removed a minute ago.  Unlike a car engine though, the Fukushima reactor is unable to dissipate the heat naturally, and so requires cooling pumps.

Most cars nowadays keep the fans running for quite some time after your turn off iginition.
Some are really loud doing that.

[ejeffrey]

It's not just the main reactor.  The spent fuel rods are kept in wet storage for some time (a few years?) after being removed from the reactor before being moved to dry storage.  Even here the decay heat is enogh to boil the water away and overhead if pumps are not running.  This was a secondary problem in the Fukushima site, although as I remember it takes days before that becomes an issue.

[CaptDon]

The generators were available but mounted at ground level and made useless after having been submerged by the first wave!! I have always thought emergency generating equipment should be inside concrete buildings and at least 20 to 30 feet above local ground level. This is how our equipment was housed at the hospital where I worked. The fuel tanks were at ground level but the vents and fillers were up at the second floor level and fuel was supplied from an access road on a hill behind the emergency generator facility. The generators operated without fail in the flood of 1972 and in 1975. Strangely enough, the hospital had a diesel generator in a sub-basement vault. It was 1MW capacity. I never figured out the logic behind that installation?

[mikerj]

Despite shutdown, the core remains hot, much like a car engine that has just had its ignition key removed a minute ago.  Unlike a car engine though, the Fukushima reactor is unable to dissipate the heat naturally, and so requires cooling pumps.

Most cars nowadays keep the fans running for quite some time after your turn off iginition.
Some are really loud doing that.

And some have DC power auxiliary coolant pumps that continue running as well.

[floobydust]

The generators were available but mounted at ground level and made useless after having been submerged by the first wave!! I have always thought emergency generating equipment should be inside concrete buildings and at least 20 to 30 feet above local ground level. This is how our equipment was housed at the hospital where I worked. The fuel tanks were at ground level but the vents and fillers were up at the second floor level and fuel was supplied from an access road on a hill behind the emergency generator facility. The generators operated without fail in the flood of 1972 and in 1975. Strangely enough, the hospital had a diesel generator in a sub-basement vault. It was 1MW capacity. I never figured out the logic behind that installation?

I have seen hotels, hospitals etc. have either above or below ground diesel generator rooms. They are kind of an afterthought on something very rarely used.

Fukushima design should not have been approved. It was due to Tepco being corrupt and greedy, they wanted it cheap and found a way to rationalize the emergency generators, switchgear etc. being so low on the ground and math that tsunami's that powerful never really happen 

A two-fault failure scenario is super dangerous, that is when people get killed and things go very bad.
Example: Combined flood and grid power outage. How are you running those sump pumps?
Or power plant I worked in, lost grid power due to switchgear explosion and the plant UPS also was dead, did not work, never gets tested beyond initial commissioning.
So that was instant lights out

[Neepa]

snip

snip....

Exactly THIS. Good old greed and some corruption coming in to ruin the promise of nuclear power. Did that ever happen previously......

AFAIK they also skimped quite heavily on emergency training and several procedures to ensure information flow; ESPECIALLY to emergency personell responding from outside the powerplant.

[richnormand]

If you are interested about the subject (and why the plant needs an auxiliary power source like batteries, diesel, connection to the power grid or a large flywheel to generate power as some used to on start-up) I would suggest you download Nucleares from Steam.
This game/simulation makes it pretty obvious how and why you need auxiliary power for the electronics, motors, turbines and such while starting the plant from an off-line situation.

[johansen]

AFAIK they also skimped quite heavily on emergency training and several procedures to ensure information flow; ESPECIALLY to emergency personell responding from outside the powerplant.

it is quite possible they could have saved the reactors had the operators known which pipe to put water into the reactor through, instead they just further flooded the basement iirc, or the concrete shell around the reactor.

no one at the site was alive when the last emergency cooling operation was done (only one test ever, at commissioning of the system 40+ years ago) (basically, just let the reactors blow off steam open to the atmosphere)

as a result, the small amount of steam escaping the building was assumed by many at the site, to be passive cooling of the reactors boiling off water, when in fact none of the water from the fire trucks made it into the reactor.

[Nominal Animal]

I find it rather telling in the sociological sense that whenever any major accidents occur, be they nuclear or otherwise, you always have individual workers risking (and voluntarily giving) their lives to save others.  However, whenever it comes to making a profit, at least some are always willing to risk workers' and other lives just to make a little bit more of that sweet, sweet profit.  Weird species.

[Circlotron]

I find it rather telling in the sociological sense that whenever any major accidents occur, be they nuclear or otherwise, you always have individual workers risking (and voluntarily giving) their lives to save others.  However, whenever it comes to making a profit, at least some are always willing to risk workers' and other lives just to make a little bit more of that sweet, sweet profit.  Weird species.

Sounds like every war since the beginning of time.

[CatalinaWOW]

I find it rather telling in the sociological sense that whenever any major accidents occur, be they nuclear or otherwise, you always have individual workers risking (and voluntarily giving) their lives to save others.  However, whenever it comes to making a profit, at least some are always willing to risk workers' and other lives just to make a little bit more of that sweet, sweet profit.  Weird species.

Unless you think Darwin was just totally off base, the evidence is that this behavior is at the very least not harmful to species survival, and there is a fair chance that it actually is good for the species.  Tough on individuals though.  And maybe our ability to create really large scale disasters is changing that equation, though the reality is a that even Chernobyl wasn't/isn't very large scale from a species point of view.

[floobydust]

It's human behaviour - the greed-driven ones have no feelings for others and could care less about viewing people as anything other than tools for profit and meat for the grinder.
Top notch engineering is always ruined by corner cutting, corrupt construction practices, and insufficient maintenance. People want quality and reliability in a nuke plant or airplane, and expect it but behind the scenes there are problems.

France has a huge nuclear power industry, I think 58 reactors. 22 of them are more than 40 years old. More than 12 were/are shut down due to stress corrosion on the cooling pipes. 32 are down right now. They want to make another 14 reactors.
"On 21 February 2022, S&P Global Ratings and Moody's downgraded the credit rating of EDF citing the technical issues at its nuclear power plants."

The outrageous construction costs of new nuke plants is the only thing preventing them from sprouting up like mushrooms, in the green energy collapse.
"French state-owned utility EDF has raised its cost estimate for the construction of six new nuclear reactors to 67.4 billion euros ($73.18 billion)" source uh that's a lot of money.

[jmelson]

A whole bunch of reasons.  First, the MASSIVE alternators for normal operation may become unstable at very light loads.  Either the RPM governor or the rotor exciter.  US nuclear plants have huge battery-powered UPS'es to cover the pump load while emergency Diesel generators start.  They have fast-start generators, but what if one of them FAILS to start?  Yikes, so they need battery backup to make sure the pumps don't stop.
The whole Chernobyl disaster was caused when a really bad reactor design (positive void coefficient) was planned to use rotational inertia to power their circulator pumps during a station blackout event.  They did a test of a new exciter design that could regulate at the load of the plant, but the alternator slowed down during the test, and so the PUMPS slowed down!  This was only part of the damn foolery that caused the disaster, but it was a part of it.  The whole idea was SO DAFT it should never have been attempted!

OH, many of the older GE boiling water plants of similar design to the Fukushima plants have a Tenney turbine, that is capable of providing sufficient cooling water a couple minutes after a reactor SCRAM to prevent meltdown, through a cooling tower on the roof.  This small turbine runs off the residual heat from the cooling reactor.  Fukushima Dai-ichi plant #1 did NOT have this feature.  All the newer plants did have it, and they worked fine until the batteries that kept the valves open went dead.
Jon

[Nominal Animal]

My point was that humans are weird: some capable of extremely altruistic actions, some of extreme exploitation, and surprisingly often both in the same human.

Note that even including Hiroshima and Nagasaki and all nuclear tests, accidents, and leaks, the radioactive particles released by coal power plants and the related death toll is still higher; and that does not include the chemical toxicity of coal power plant pollution at all.
(Which reminds me: depleted uranium is amazingly toxic heavy metal, more dangerous than lead or mercury fumes.  When you get enough uranium in your body to kill you, it is the chemical toxicity and not the radiation that kills you first.)

A big problem with nuclear plants is that they just want to build them way too large; simply because the return on the investment takes a couple of decades to realize, and people feel the return must be maximized by scale.  It's the age-old "I want my profit now, or at the end of this quarter at the latest" problem.
I like the way it is said Romans commissioned their bridges: half when the bridge was finished, the other half fifty years later if the bridge was still standing.

A question one can ask is why don't we build the reactors where we intend to store the radioactive waste long-term, a mile or two beneath the surface, at least half a mile below groundwater deposits.  Such repositories already have to be accounted for the total price of nuclear energy, so why not build them there in the first place.

[uer166]

My point was that humans are weird: some capable of extremely altruistic actions, some of extreme exploitation, and surprisingly often both in the same human.

Note that even including Hiroshima and Nagasaki and all nuclear tests, accidents, and leaks, the radioactive particles released by coal power plants and the related death toll is still higher; and that does not include the chemical toxicity of coal power plant pollution at all.
(Which reminds me: depleted uranium is amazingly toxic heavy metal, more dangerous than lead or mercury fumes.  When you get enough uranium in your body to kill you, it is the chemical toxicity and not the radiation that kills you first.)

A big problem with nuclear plants is that they just want to build them way too large; simply because the return on the investment takes a couple of decades to realize, and people feel the return must be maximized by scale.  It's the age-old "I want my profit now, or at the end of this quarter at the latest" problem.
I like the way it is said Romans commissioned their bridges: half when the bridge was finished, the other half fifty years later if the bridge was still standing.

A question one can ask is why don't we build the reactors where we intend to store the radioactive waste long-term, a mile or two beneath the surface, at least half a mile below groundwater deposits.  Such repositories already have to be accounted for the total price of nuclear energy, so why not build them there in the first place.

How the heck are you going to cool and service a reactor miles underground?? There's a good reason most/all reactors are next to a large body of water.

[floobydust]

[...] US nuclear plants have huge battery-powered UPS'es to cover the pump load while emergency Diesel generators start.  They have fast-start generators, but what if one of them FAILS to start?  Yikes, so they need battery backup to make sure the pumps don't stop. [...]

That's not how it is in the coal plants, the UPS doesn't have enough power for the boiler feed and cooling pumps.
The plant UPS is strictly for SCADA visibility and the computers, the Control Room needs those monitors so they can tell what's going on.

It's a good point relevant to OP is the power requirements for the pumps.
Steam loop, the boiler feed pumps are huge for the 15MPa, 100's HP as I remember. We had one one bag due to PLC software mistake.

[johansen]

How the heck are you going to cool and service a reactor miles underground?? There's a good reason most/all reactors are next to a large body of water.

Well for one you dont have to pump water, just let it fall down the shaft

[Njk]

It's a good point relevant to OP is the power requirements for the pumps.
Steam loop, the boiler feed pumps are huge for the 15MPa, 100's HP as I remember. We had one one bag due to PLC software mistake.

As an example, one unit at a modern NPP generates 1.2 GW of electric power, 7.5% of which is required for its own needs. So each unit requires 90 MW, from internal or external source. Typically, there are several units at NPP.

[Nominal Animal]

A question one can ask is why don't we build the reactors where we intend to store the radioactive waste long-term, a mile or two beneath the surface, at least half a mile below groundwater deposits.  Such repositories already have to be accounted for the total price of nuclear energy, so why not build them there in the first place.

How the heck are you going to cool and service a reactor miles underground?? There's a good reason most/all reactors are next to a large body of water.

Cooling is an interesting question, because thermal-induced vertical flows in water are pretty strong and effective, and cool water is a lot heavier than hot water.  You'd probably use large vertical tubes to surface, which would also mean quite high water pressure at the reactor, unless intermediate pumping stations and water reservoirs would be used –– which in turn could be used for kinetic and thermal energy storage from solar and wind power too.

As to maintenance: The same way you maintain a mine, or service anything related to large city infrastructure.  At least here, they tend to be underground already.  (In central Helsinki, about 100m underneath the Esplanadi park, there is a 8m wide, 40m tall, 80m long artificial "lake" with 25,000 cubic meters of water solely for district cooling.)
There is some extra cost involved in transporting stuff, as you use long spiraling ramps, not elevators, so it's comparable to having it say 30 miles outside your major city.

It really is much less crazy than might appear at first, especially for experienced civil engineers.

[sparkydog]

How the heck are you going to cool and service a reactor miles underground?? There's a good reason most/all reactors are next to a large body of water.

Well for one you dont have to pump water, just let it fall down the shaft

Great idea! Now your plant is flooded with hot water. (Although, in theory, the pressure of the incoming water should make it easier to pump the water back out, and the outgoing water being hot might mean you get some assistance from convection. But you'd still need to pump the water, which means pipes, and pipes to carry that much water are expensive.)

[5U4GB]

Using Fukushima as an example.  Immediately upon earthquake detection, all the control rods dropped into place and shutdown the nuclear reaction and power generation.

Fukushima is a boiling water reactor not a pressurised water reactor so things work differently there, the control rods are inserted from underneath because the top half is used for steam separation and dryers, so there's no gravity-assisted way to insert them, you need power. Also since the control rod mechanisms are at the lowest part of the reactor building, if it floods...

[5U4GB]

It's not just the main reactor.  The spent fuel rods are kept in wet storage for some time (a few years?) after being removed from the reactor before being moved to dry storage.  Even here the decay heat is enogh to boil the water away and overhead if pumps are not running.  This was a secondary problem in the Fukushima site, although as I remember it takes days before that becomes an issue.

It's also a major problem at Sellafield in the UK, google "First Generation Magnox Storage Pond" for details.

[5U4GB]

it is quite possible they could have saved the reactors had the operators known which pipe to put water into the reactor through, instead they just further flooded the basement iirc, or the concrete shell around the reactor.

If you're referring to the high pressure coolant injection system (HPCI) which was never activated, that's an interesting one.  What that would do is inject cold water onto a very hot containment vessel that's gone through decades of radiation embrittlement (the impact of neutrons creates voids that make the reactor vessel more and more brittle over time), the TMI operators actually shut it down after a few minutes out of concern that the thermal shock would cause a breach of the primary containment.  Nuclear physicists are divided 50:50 on whether this was a good thing or a bad thing.  I certainly wouldn't want to be anywhere near a reactor where such an experiment was performed.

[johansen]

it is quite possible they could have saved the reactors had the operators known which pipe to put water into the reactor through, instead they just further flooded the basement iirc, or the concrete shell around the reactor.

If you're referring to the high pressure coolant injection system (HPCI) which was never activated, that's an interesting one.  What that would do is inject cold water onto a very hot containment vessel that's gone through decades of radiation embrittlement (the impact of neutrons creates voids that make the reactor vessel more and more brittle over time), the TMI operators actually shut it down after a few minutes out of concern that the thermal shock would cause a breach of the primary containment.  Nuclear physicists are divided 50:50 on whether this was a good thing or a bad thing.  I certainly wouldn't want to be anywhere near a reactor where such an experiment was performed.

There is a lengthy youtube vid on this. They didnt know what valves were open or closed for various reasons.  They should have been able to do it, would have made a cubic mile sized steam cloud, but they assumed it was working because of a small steam cloud.

Sorry trying to simplify it..

[jmelson]

[...] US nuclear plants have huge battery-powered UPS'es to cover the pump load while emergency Diesel generators start.  They have fast-start generators, but what if one of them FAILS to start?  Yikes, so they need battery backup to make sure the pumps don't stop. [...]

That's not how it is in the coal plants, the UPS doesn't have enough power for the boiler feed and cooling pumps.
The plant UPS is strictly for SCADA visibility and the computers, the Control Room needs those monitors so they can tell what's going on.

It's a good point relevant to OP is the power requirements for the pumps.
Steam loop, the boiler feed pumps are huge for the 15MPa, 100's HP as I remember. We had one one bag due to PLC software mistake.

The only plant I have been inside was the Calloway County plant in central Missouri.  It is a pressurized water reactor.  The circulation pumps are inside the containment, along with the steam generators.  This keeps the primary coolant loop very small, for safety reasons.  There are 4 pumps and 4 steam generators, each pump has a 1000 Hp motor.  If a steam generator develops a leak, it can be valved off and they can run  on only 3.
Jon

[WatchfulEye]

Fukushima is a boiling water reactor not a pressurised water reactor so things work differently there, the control rods are inserted from underneath because the top half is used for steam separation and dryers, so there's no gravity-assisted way to insert them, you need power. Also since the control rod mechanisms are at the lowest part of the reactor building, if it floods...

However, this can be done hydraulically. Hydraulic pressure can be stored in accumulator pressure vessels. Therefore all that is needed is to open the valve from accumulator to hydraulic rams and rod insertion will proceed.

You don't even need an oil hydraulic system - you can use the reactor coolant itself as hydraulic fluid. This means that force on the rams is proportional to reactor vessel pressure - with the correct sized hydraulic pistons and mechanical advantage, it is possible to guarantee that the insertion force will exceed hydrostatic pressure in the vessel.

Add a normally open hydraulic valve, and if power is lost, or the control system output is lost, the valve opens and the rods are inserted with no additional energy or control input.

[floobydust]

[...] US nuclear plants have huge battery-powered UPS'es to cover the pump load while emergency Diesel generators start.  They have fast-start generators, but what if one of them FAILS to start?  Yikes, so they need battery backup to make sure the pumps don't stop. [...]

That's not how it is in the coal plants, the UPS doesn't have enough power for the boiler feed and cooling pumps.
The plant UPS is strictly for SCADA visibility and the computers, the Control Room needs those monitors so they can tell what's going on.

It's a good point relevant to OP is the power requirements for the pumps.
Steam loop, the boiler feed pumps are huge for the 15MPa, 100's HP as I remember. We had one one bag due to PLC software mistake.

The only plant I have been inside was the Calloway County plant in central Missouri.  It is a pressurized water reactor.  The circulation pumps are inside the containment, along with the steam generators.  This keeps the primary coolant loop very small, for safety reasons.  There are 4 pumps and 4 steam generators, each pump has a 1000 Hp motor.  If a steam generator develops a leak, it can be valved off and they can run  on only 3.
Jon

I'm trying to get some idea of the "Residual Heat Removal (RHR)" pumps' required power.  There's a few different cooling loops, it's complicated. Reactor cooling is not in the steam (generation) loop so the high feed pressure is not a requirement I believe.
But still 1,000's GPH flow. RHR seems to run for 4 hours min. from 350°F and 425 psig but then it says "The design heat load is based on the decay heat fraction that exists at 20 hours following reactor shutdown from extended operations at full power." That's all in the diesel generator league, not battery/UPS or hamsters running in a wheel.

"The RHR system transfers heat from the reactor coolant system to the component cooling water system. During shut down plant operations, the RHR system is used to remove the decay heat from the core and reduce the temperature of the reactor coolant to the cold shutdown temperature (< 200°F). The cooldown performed by the RHR system (from 350°F to < 200°F) is referred to as the second phase of cooldown. The first phase of cooldown is accomplished by the auxiliary feedwater (AFW) system (Chapter 5.8, steam dump control system (Chapter 11.2), and the steam generators.

Westinghouse Technology Systems Manual, Section 5.1, Residual Heat Removal (RHR) System
Westinghouse Technology Systems Manual, Section 5.2, Emergency Core Cooling Systems

Antique "Fukushima Daiichi reactors are GE boiling water reactors (BWR) of an early (1960s) design supplied by GE, Toshiba and Hitachi, with what is known as a Mark I containment." has process diagram.

[Terry Bites]

Where's the power going to come from when the reactor is shut down, from the grid.
Off on a bit of a tangent. I did some work at big old coal fired station.
They had an enormous battery room. A beautiful tiled floor covered in glass sided cells over a metre high. Fab, I should have taken a photo.

[jmelson]

You don't even need an oil hydraulic system - you can use the reactor coolant itself as hydraulic fluid. This means that force on the rams is proportional to reactor vessel pressure - with the correct sized hydraulic pistons and mechanical advantage, it is possible to guarantee that the insertion force will exceed hydrostatic pressure in the vessel.

Add a normally open hydraulic valve, and if power is lost, or the control system output is lost, the valve opens and the rods are inserted with no additional energy or control input.

As far as I know, all (water-cooled) reactors use WATER hydraulic systems to operate the control rods.  Advantages are they have lots of high-pressure water pumps already there, and if there's a leak, it doesn't inject oil into the reactor coolant.
Jon

[Ground_Loop]

Primary loop pumps are used to heat the reactor prior to startup, so an external power source is required.  Once the reactor is on line, local steam gen sets can be paralleled with the outside grid and switched in to power the pumps.  Further, upon shutdown, decay heat needs to be removed for weeks to months afterwards which requires loop circulation.  I was a reactor operator on an aircraft carrier.